In known flat keys with recesses for receiving the preferably cylindrical tumbler pins located in radially displaceable manner in the lock rotor, the recesses are drilled out of the flat side of the key, so that conical countersunk hole are formed on the key shank. Thus, the largest diameter on the key surface of each hole is a function of the depth of the particular hole, i.e., the larger the depth or step value of the particular recess selected corresponding to the lateral permutation of the flat key, the larger the diameter.
In principal a corresponding relationship between the depth and width of the key recess also exists if, in order to conceal the actual position of the tumbler pins and consequently making unauthorized duplication of the key more difficult, the recesses are formed so that they are elongated in the longitudinal direction of the key, i.e., are not drilled out but are instead milled into the key shank or blade with a milling cutter of the same shape and size as the drill for the above-mentioned countersunk holes. The reason is that the largest width of a milled ablong recess measured on the key surface at right angles to the key shank corresponds, for a milled recess of the same depth as for the countersunk hole to the largest diameter of the recess made as a conical countersunk hole on the key surface.
A recess made in the form of a countersunk hole takes up just as much space longitudinally and transversely relative to the key shank, although in the case of drilled recesses with larger step depths in which the cylindrical portion of the tumbler pin engaged in the recess is located below the key surface, the space requirement at right angles to the key could be smaller because the diameter of the associated cylindrical tumber pins in considerably smaller than the largest diameter of the conical countersunk hole measured on the key surface. This largest bore diameter is, in fact, necessary in order to form cam or slide surfaces for the tumbler pins on the sides of the recess which extend in the longitudinal direction of the key up to the surface thereof and which are preferably inclined at an angle of 45.degree. thereto. Thus, on removing the key, the pin tips thereof can perfectly ascend these side portions up to the surface of the key.
In connection with the space requirement in the area of the key surface, fundamentally the same applies for the oblong milled recesses as for the conical countersunk holes, but in this case the excessively large space requirement at right angles to the key is an even greater disadvantage and specifically when the flat key is used as a turning key with a double recess design.
In the case of flat keys, it is also known to permit the recesses formed lengthwise for concealing the tumbler positions and successively milled in a row in the longitudinal direction of the key to pass into one another, whereby then the slides for the tumbler pins do not, or do not all, extend up to the key surface, but instead depending on the depth difference of successive step depths, differ in length. However, here again at least in the case of the recesses with the greatest step depth of the 90.degree. later permutation, the disadvantage of the excessively large space requirement at right angles to the key shank still exists (Swiss Pat. No. 260,517).
In addition, the excessively large three-dimensional space requirement, particularly of milled oblong recesses of the 90.degree. lateral permutation, makes it impossible to use smaller key thicknesses with a reduced shank cross-section relative to the width and thickness such as would be desirable for use with lock cylinders with smaller diameters while retaining the number of permutations. In addition, due to the excessively large width of the recess at right angles to the key, the tumbler pins for an edge permutation to be provided on the narrow side of the key can only have a correspondingly reduced diameter or only a restricted insertion depth.
A further disadvantage of the hitherto known methods for manufacturing flat keys with longitudinally milled recesses is that they are either very complicated or in the case of profile milling machines with a plurality of juxtaposed control slide valves are too imprecise, regarding the precision of the slides on the recesses, for the desired clean ascent of the tumbler pins on removing the key.